Long Stroke Riser Tensioner System and Wellbay Structure for a Floating Unit

ABSTRACT

A riser tensioner system and a wellbay structure for a floating unit or platform for deep/ultra-deep water field development. The riser tensioner system includes a first cassette, a second cassette, a tension joint, one or more centralizers, and a plurality of cylinders. The one or more centralizers provide lateral support to the tension joint. Each of the plurality of cylinders comprises a first end, a second end, and an intermediate portion. The first end of each of the plurality of cylinders is secured to the first cassette, and an intermediate portion of each of the plurality of cylinders is secured to the second cassette. The wellbay structure includes a plurality of transverse girders and a plurality of longitudinal girders. The girders form a grid comprising a plurality of slots. Each of the plurality of slots is configured for receiving and supporting a first cassette of each a riser tensioner assembly.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 62/115,994, entitled “Long Stroke Riser Tensioner System and Wellbay Structure for a Floating Production and/or Drilling Unit” and filed Feb. 13, 2015, the contents of which application are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a floating unit or platform and, more specifically, to a semi-submersible floating platform comprising a low-profile wellbay structure and at least one long stroke riser assembly disposed within the wellbay structure, the at least one long-stroke riser assembly comprising at least one long-stroke riser and a respective top-mounted tensioner system for the at least one long-stroke riser for deep water and ultra-deep water field development.

BACKGROUND OF THE INVENTION

Offshore exploration has been moving into ultra-deepwater, especially for subsalt and lower tertiary reservoirs demanding large topside facilities with drilling capability. Floating system platforms, such as tension-leg platforms (TLPs) and spar platforms, encounter several disadvantages that may limit their use in ultra-deepwater. Due to technical and/or commercial challenges, TLPs are limited by water depth. Spar platforms may be constrained by topsides capacity, and they pose challenges in offshore installation in deep water (e.g., water having a depth between 1500 feet and 5000 feet, inclusive) and ultra-deep water (e.g., water having a depth greater than 5000 feet).

Although many floating systems could be used for dry tree solutions, there are substantial challenges to create a feasible and cost-effective solution for deep and ultra-deep water field development with dry trees. For a semi-submersible, an important challenge is the vertical or heave motion characteristics of the floater, which poses a significant impact on riser performance and demand a change of the hull and/or riser system design to compensate top-tensioned riser stroke motion.

Some long-stroke riser tensioners with stroke range up to 28 feet have been successfully used on production and drilling platforms for accommodating the vertical or heave motion characteristics of the floater. A 28-foot stroke range may be insufficient in some deep water and ultra-deep water applications. Increasing the stroke range further presents numerous design challenges, such as maintaining a sufficient air-gap in the vessel while not increasing its height too much to minimize the wind force on the vessel, providing stability to the long-stroke riser tensioners, etc. Furthermore, as the tensioner stroke range increases, the problems of cylinder rod loading (in compression with potential buckling), eccentric loading when one cylinder is out of service, trash getting into the cylinder head seals, and lateral side loading on the cylinder rods increase.

Floating system platforms, such as tension-leg platforms (TLPs) and spar platforms, have been used to support top-tensioned risers for dry tree solutions for several decades. For fabrication and operational reasons, a conventional floating vessel includes a wellbay and will normally have its top flanges flush with the lower deck elevation (normally at approximately 5 or more feet above the topside deck underside). The purpose is to avoid deck elevation step up coming in from surrounding areas. This means that the wellbay structure girders could protrude as much as 5 to 7 feet below the lower deck of the floater and could influence the air gap design for the entire topside.

SUMMARY OF THE INVENTION

In accordance with an aspect of the present invention, there is provided a riser tensioner system including a first cassette, a second cassette, a tension joint, one or more centralizers, and a plurality of cylinders. The tension joint is slidably disposed through the first cassette and the second cassette. The one or more centralizers provide lateral support to the tension joint. Each of the plurality of cylinders comprises a first end, a second end, and an intermediate portion. The first end of each of the plurality of cylinders is secured to the first cassette, and the intermediate portion of each of the plurality of cylinders is secured to the second cassette.

In accordance with another aspect of the present invention, there is provided a wellbay structure for use in an offshore platform. The wellbay structure includes a plurality of transverse girders and a plurality of longitudinal girders. The plurality of transverse girders and the plurality of longitudinal girders form a grid comprising a plurality of slots. Each of the plurality of slots is configured for receiving and supporting a first cassette of each of a respective one of a plurality of riser tensioner assemblies.

In accordance with yet another aspect of the present invention, there is provided an offshore platform comprising a wellbay structure and at least one riser tensioner assembly. The wellbay structure comprises a plurality of transverse girders and a plurality of longitudinal girders. The at least one riser tensioner assembly comprises a first cassette, a second cassette, a tension joint, one or more centralizers, and a plurality of cylinders. The tension joint of the at least one riser tensioner assembly is slidably disposed through the first cassette and the second cassette. The one or more centralizers provide lateral support to the tension joint. Each of the plurality of cylinders of each of the plurality of riser tensioner assemblies comprises a first end, a second end, and an intermediate portion. The first end of each of the plurality of cylinders is secured to the first cassette, and the intermediate portion of each of the plurality of cylinders is secured to the second cassette. The plurality of transverse girders and the plurality of longitudinal girders of the wellbay structure form a grid comprising a plurality of slots. At least one of the plurality of slots is configured for receiving and supporting the first cassette.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustration, there are shown in the drawings certain embodiments of the present invention. In the drawings, like numerals indicate like elements throughout. It should be understood that the invention is not limited to the precise arrangements, dimensions, and instruments shown. In the drawings:

FIG. 1 illustrates a perspective view of a semi-submersible floating vessel comprising a deck structure comprising an upper level and a lower level, in accordance with an exemplary embodiment of the present invention;

FIG. 2 illustrates an overall plan view of the lower level of the deck structure of the vessel of FIG. 1, the lower level having a wellbay area comprising a wellbay disposed therein, in accordance with an exemplary embodiment of the present invention;

FIG. 2A illustrates a plan view of the wellbay area of FIG. 2 disposed within the deck of the vessel of FIG. 1, in accordance with an exemplary embodiment of the present invention;

FIG. 2B illustrates a perspective view of the wellbay area of the vessel of FIG. 1, in accordance with an exemplary embodiment of the present invention;

FIG. 2C illustrates a cross-sectional view of the wellbay area of the vessel of FIG. 1, in accordance with an exemplary embodiment of the present invention;

FIG. 3 illustrates a cross-section of a portion of the vessel of FIG. 1 showing an exemplary embodiment of a riser assembly from the wellbay to a wellhead at seafloor, the riser assembly comprising a tensioner system, in accordance with an exemplary embodiment of the present invention;

FIG. 4 illustrates an exemplary embodiment of the tensioner system of FIG. 3, the exemplary embodiment of the tensioner system comprising a centralizing pipe sleeve and a plurality of centralizers, in accordance with an exemplary embodiment of the present invention;

FIG. 4A illustrates a cross-sectional view of the tensioner system of FIG. 3 and specifically a cross sectional view of the centralizing pipe sleeve, in accordance with an exemplary embodiment of the present invention;

FIGS. 4B and 4C illustrate views of an exemplary embodiment of a tension joint and a layout of exemplary embodiments of centralizers, respectively, the tension joint replacing the centralizing pipe sleeve illustrated in FIG. 4 and the centralizers replacing the centralizers of FIG. 4, in accordance with an exemplary embodiment of the present invention;

FIGS. 4D through 4F illustrate close-up views of components of the tensioner assembly of FIG. 3, in accordance with an exemplary embodiment of the present invention;

FIGS. 5A and 5B illustrate perspective view of the wellbay disposed within the deck illustrated in FIG. 2, in accordance with an exemplary embodiment of the present invention;

FIG. 5C illustrates a perspective view of a slot of the wellbay of FIG. 2 and a cassette of a riser tensioner system configured to be disposed in the slot, in accordance with an exemplary embodiment of the present invention

FIG. 5D illustrates a perspective view of the cassette of FIG. 5C disposed within the slot of FIG. 5C, in accordance with an exemplary embodiment of the present invention;

FIGS. 6A and 6B illustrate a first embodiment of a tensioner lateral stabilization system, in accordance with an exemplary embodiment of the present invention; and

FIG. 6C illustrates a second embodiment of a tensioner lateral stabilization system, in accordance with an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference to the drawings illustrating various views of exemplary embodiments of the present invention is now made. In the drawings and the description of the drawings herein, certain terminology is used for convenience only and is not to be taken as limiting the embodiments of the present invention. Furthermore, in the drawings and the description below, like numerals indicate like elements throughout.

Referring now to FIG. 1, there is illustrated a perspective view of a semi-submersible floating vessel, generally designated as 1000, for extracting oil and gas from a subsea reservoir, in accordance with an exemplary embodiment of the present invention. The vessel 1000 comprises a hull 1010 comprising a deck 2000, a bottom level structure 1020, and a plurality of columns 1030A, 1030B, 1030C, and 1030D (not illustrated) connecting the deck 2000 to the bottom level structure 1020. The deck 2000 comprises a deck structure 2005 and a wellbay 5000 comprising a structure 5020 in which riser tensioners 4000 (illustrated in FIG. 2B) are disposed. The bottom level structure 1020 comprises a plurality of hull pontoons 1022A, 1022B, 1022C, and 1022D and a riser keel guide support structure 1024. The vessel 1000 is semi-submersible and is chosen to have a hull draft that is suitable for quayside construction requirements and deep water/ultra-deep water oil and gas extraction operations.

Illustrated in FIG. 2 is a plan view of the deck 2000, and illustrated in FIG. 2B is a perspective view of a wellbay 5000 of the deck 2000, in accordance with an exemplary embodiment of the present invention. Referring to FIGS. 2 and 2B, the deck structure 2005 of the vessel 1000 comprises a top 2001 and a bottom 2003. The wellbay structure 5020 is a structural support grid having a top surface 5001 and a bottom surface 5003. The wellbay structure 5020 (also referred to herein as the “structural support grid 5020”) is configured to support a plurality of production riser assemblies 3000B-F, I, M, and P-T and a drilling riser 2015.

The wellbay 5000 is located centrally in the vessel 1000 to allow the production riser assemblies 3000B-F, I, M, and P-T to be centrally located. The deck structure 2005 in wellbay 5000 comprises two large beams 2030A and 2030B that traverse the length of the deck structure 2005 along with two rows of columns 2030C-H, J-K that support a drilling unit 1100 (illustrated in FIG. 2C) via a drilling support structure 1110 (also illustrated in FIG. 2C). Disposed between the columns 2030C-H, J-K (not shown) is structural bracing. The wellbay 5000 is disposed between the beams 2030A and 2030B, which provide segregation of the wellbay 5000 from the areas adjacent to the outsides of the beams 2030A and 2030B. In an exemplary embodiment, the bottom surface 5003 of the wellbay structure 5020 may be flush with the bottom 2003 of the deck structure 2005.

To facilitate operation of the vessel 1000 in deep/ultra-deep water, in accordance with an exemplary embodiment of the present invention, the riser assemblies 3000B-F, I, M, and P-T are long-stroke riser assemblies. The deck 2000 of the vessel 1000 further comprises banks 2020A and 2020B of high pressure accumulators for the respective riser assembly 3000B-F, I, M, and P-T. The banks of high pressure accumulators 2020A and 2020B are located adjacent to but outside of the wellbay 5000 for safety and to provide for a safer and less congested working area within the wellbay 5000.

Illustrated in FIG. 2A, is a close-up plan view of the wellbay 5000 of the vessel 1000, in accordance with an exemplary embodiment of the present invention. As illustrated, there are 12 riser assemblies 3000B-F, I, M, and P-T disposed within the wellbay 5000 of the vessel 1000. Attached to the top of each riser assembly 3000B-F, I, M, and P-T is a respective valve block (also known as a “Christmas tree”) 2010B-F, I, M, and P-T. By virtue of their connection to the tops of the riser assemblies 3000B-F, I, M, and P-T, the valve blocks 2010B-F, I, M, and P-T are top-mounted surface valve blocks, i.e., dry trees.

Referring now to FIG. 3, there is illustrated a cross-section of a portion of the vessel 1000, showing an exemplary embodiment of each of the riser (production) assemblies 3000B-F, I, M, and P-T, in accordance with an exemplary embodiment of the present invention. As illustrated in FIG. 3, each riser assembly 3000B-F, I, M, and P-T comprises a respective top-tensioned riser 3010B-F, I, M, and P-T and a respective top-mounted tensioner 4000B-F, I, M, and P-T (also referred to herein as “tensioner system 4000B-F, I, M, and P-T”). Each top-tensioned riser 3010B-F, I, M, and P-T conveys oil and gas from the seafloor 300. Each valve block 2010B-F, I, M, and P-T at the top the respective riser assembly 3000B-F, I, M, and P-T controls the conveyance of oil and gas through the respective top-tensioned riser 3010B-F, I, M, and P-T. Each top-mounted tensioner 4000B-F, I, M, and P-T applies tension to its respective top-tensioned riser 3010B-F, I, M, and P-T. Each riser 3010B-F, I, M, and P-T comprises a respective outer casing 3015B-F, I, M, and P-T, an inner casing (not illustrated) for a dual-casing riser configuration only (if there is any), and tubing, which conveys the oil and gas from the seafloor 300.

Each top-tensioned riser 3010B-F, I, M, and P-T is supported at its top 3011B-F, I, M, and P-T by the deck 2000 (via an interface with the wellbay structural support grid 5020) of the vessel 1000. An intermediate section of each top-tensioned riser 3010B-F, I, M, and P-T is embedded within a respective intermediate section (keel joint) 3012B-F, I, M, and P-T interfacing with a respective riser keel guide 1025B-F, I, M, and P-T to constrain the lateral displacement of the each top-tensioned riser 3010B-F, I, M, and P-T relative to the vessel 1000. At the bottom 3013B-F, I, M, and P-T of each top-tensioned riser 3010B-F, I, M, and P-T is a respective riser joint 3020B-F, I, M, and P-T. The bottom 3013B-F, I, M, and P-T of the top-tensioned riser 3010B-F, I, M, and P-T is connected to a tapered stress joint 3030B-F, I, M, and P-T, which is secured to a respective well head 3050B-F, I, M, and P-T in the seafloor 300 by a respective subsea connector 3040B-F, I, M, and P-T.

In order to maintain stress of the risers 3010B-F, I, M, and P-T under allowable design values at various loading conditions, the top-mounted tensioners 4000B-F, I, M, and P-T (also referred to as a “riser motion compensation systems” herein) are coupled to the respective tops 3011B-F, I, M, and P-T of the respective riser assemblies 3000B-F, I, M, and P-T. An important feature of each tensioner system 4000B-F, I, M, and P-T is its stroke length, which determines how much stroke of the respective riser 3010B-F, I, M, and P-T will be compensated by each tensioner system 4000B-F, I, M, and P-T with relative soft stiffness. Consequently, the strength and fatigue performance of each top-tensioned riser 3101B-F, I, M, and P-T will be governed by the configuration its respective tensioner system 4000B-F, I, M, and P-T and mechanical properties thereof.

Each top-tensioned riser 3010B-F, I, M, and P-T, specifically the keel joint 3012B-F, I, M, and P-T thereof, is laterally restrained below the surface 351 of the water 350 at the respective keel guide 1025B-F, I, M, and P-T by the keel guide support 1024 at the hull pontoon 1022 level of the vessel 1000. The keel guides 1025B-F, I, M, and P-T act as guides for the vertical movement of their respective risers 3010B-F, I, M, and P-T. They also withstand lateral and bending forces imposed on the risers 3010B-F, I, M, and P-T to protect the tensioner systems 4000B-F, I, M, and P-T.

Illustrated in FIG. 4 is a perspective view of each tensioner system 4000B-F, I, M, and P-T, in accordance with an exemplary embodiment of the present invention. Illustrated in FIG. 4D is a close-up perspective view of the interface between each tensioner system 4000B-F, I, M, and P-T and the wellbay 5000 structural support grid 5020 (illustrated in FIG. 2B and FIG. 2C), in accordance with an exemplary embodiment of the present invention. Illustrated in FIG. 4E is a close-up perspective view of the top of each tensioner system 4000B-F, I, M, and P-T, in accordance with an exemplary embodiment of the present invention.

Referring now to FIGS. 4, 4D, and 4E, each tensioner system 4000B-F, I, M, and P-T comprises a plurality of cylinders 4010A through 4010F, a load ring 4020, an upper cassette 4040, and a lower cassette 4050. The plurality of cylinders 4010A-F are connected to the high pressure accumulators 2020A and 2020B (illustrated in FIG. 2) and low-pressure accumulators adjacent to the tensioner systems 4000B-F, I, M, and P-T. Each tensioner system 4000B-F, I, M, and P-T is designed to transfer tension force from its respective vertical riser 3010B-F, I, M, and P-T back into the wellbay 5000 structural support grid 5020. Desirably, each tensioner system 4000B-F, I, M, and P-T works independently from one other. Failure or malfunction of one tensioner system 4000B-F, I, M, and P-T will not affect the other tensioner systems 4000B-F, I, M, and P-T.

For each of the tensioner systems 4000B-F, I, M, and P-T, a top portion 4011A through 4011F of each respective cylinder 4010A through 4010F is secured to the upper cassette (herein also referred to as a “support frame”) 4040. A middle portion 4012A through 4012F of each respective cylinder 4010A through 4010F is secured to the lower cassette 4050. A lower portion 4013A through 4013F of each respective cylinder 4010A through 4010F is over hung below the lower cassette 4050. The support frame 4040 supports the cylinders 4010A through 4010F and transfers the riser force from its respective vertical riser 3010B-F, I, M, and P-T back into the wellbay 5000. The cylinders 4010A-F are disposed in a lower portion 4003 of each tensioner system 4000B-F, I, M, and P-T.

In the exemplary embodiment illustrated in FIG. 4, the tensioner systems 4000B-F, I, M, and P-T each comprise six cylinders 4010A through 4010F. It is to be understood that the tensioner system 4000 is not limited to having six cylinders 4010A through 4010F. For example, it is contemplated that the tensioner systems 4000B-F, I, M, and P-T may have four cylinders, eight cylinders, etc. in other exemplary embodiments. The tensioner systems 4000B-F, I, M, and P-T comprise more than one cylinder 4010A-F to allow for the loss or removal for maintenance of one of the cylinders 4010A-F without hindering performance of the tensioner systems 4000B-F, I, M, and P-T.

Each cylinder 4010A-F includes a respective extendible and retractable cylinder rod 4015A through 4015F. The cylinder rods 4015A-F are designed with sufficient capacity to handle maximum buckling loads expected during in situ operation of the vessel 1000. In an exemplary embodiment, each of the cylinder rods 4015A-F has a stroke length of up to 50 feet.

Each tensioner system 4000B-F, I, M, and P-T further comprises a tension joint 4090. The tension joint 4090 interface with the upper cassette 4040 and the lower cassette 4050 to prevent the tension joint 4090 and the load ring 4020 from rotating relative to the upper cassette 4040 and the lower cassette 4050, thereby assuring that the top ends of the cylinder rods 4015A through 4015F do not rotate. Rotation of the top ends of the cylinder rods 4015A-F would severely damage the cylinders 4010A-F and the cylinder rods 4015A-F. A top portion 4091 of the tension joint 4090 is centered within the load ring 4020 by a positioning ring 4031 and secured to the load ring 4020. A connector 4032 secures the top portion 4091 of the tension joint 4090 to its respective riser 3010B-F, I, M, and P-T, which passes through the tension joint 4090 of its respective tensioner system 4000B-F, I, M, and P-T. A bottom portion 4093 of the tension joint 4090 is secured to its respective riser 3010B-F, I, M, and P-T by a connector 4033. Each tensioner system 4000B-F, I, M, and P-T comprises an upper portion 4001 comprising the load ring 4020, the positioning ring 4031, the connector 4032, and a top portion 4091 of the tension joint 4090.

In an exemplary embodiment, illustrated in FIGS. 4 and 4A, the tension joint 4090 of each tensioner system 4000B-F, I, M, and P-T is a centralizing pipe sleeve which surrounds a special portion of the riser outer casing 3015B-F, I, M, and P-T in the respective tensioner system 4000B-F, I, M, and P-T. The centralizing pipe sleeve 4090 comprises at least two guide rails 4095A and 4095B disposed on an outer surface of the centralizing pipe sleeve 4090. Disposed on the upper cassette 4040 are a plurality of upper centralizers 4060A through 4060D, and disposed on the lower cassette 4050 are a plurality of lower centralizers 4070A through 4070D. The guide rail 4095A is disposed against the centralizers 4060C, 4070C and 4060D, 4070D, and the guide rail 4095B is disposed against the centralizers 4060A, 4070A and 4060B, 4070B to prevent the centralizing pipe sleeve 4090 and the load ring 4020 from rotating or moving laterally relative to the upper cassette 4040 and the lower cassette 4050. In such embodiment, the centralizing pipe sleeve 4090 is connected to the outer casing 3015B-F, I, M, and P-T at the bottom portion 4093 of the centralizing pipe sleeve 4090 via the connector 4033.

In another exemplary embodiment, illustrated in FIGS. 4B and 4C, the tension joint 4090 of each tensioner system 4000B-F, I, M, and P-T is a special portion of the riser outer casing 3015B-F, I, M, and P-T in the respective tensioner system 4000B-F, I, M, and P-T. Such exemplary embodiment is illustrated in FIG. 4B and is generally designated as 4090′. The tension joint 4090′ has tension joint upsets (also referred to as “tension joint protrusions”) 4095A′ through 4095D′. The tension joint upsets 4095A′-4905D′ interface with a slot guide plate 4042 on the upper cassette 4040 and a slot guide plate 4072 on the lower cassette 4050, illustrated in FIG. 4C, to prevent the tension joint 4090′ and the load ring 4020 from rotating relative to the upper cassette 4040 and the lower cassette 4050, thereby assuring that the top ends of the cylinder rods 4015A through 4015F do not rotate. Specifically, the tension joint upsets 4095A′-4905D′ interface with slots 4043A through 4043D in the slot guide plate 4042 and with slots 4073A through 4073D in the slot guide plate 4072 to prevent the tension joint 4090′ and the load ring 4020 from rotating relative to the upper cassette 4040 and the lower cassette 4050. Rotation of the top ends of the cylinder rods 4015A-F would severely damage the cylinders 4010A-F and the cylinder rods 4015A-F. A top portion 4091 of the tension joint 4090′ is centered within the load ring 4020 by a positioning ring 4031 and secured to the load ring 4020. The centralizers 4060A-D and 4070A-D further help to prevent lateral deflection of the tension joint 4090′. The tension joint 4090′ is connected to the remainder of the outer casing 3015B-F, I, M, and P-T via the connector 4033. It is to be understood that the tension joint 4090′ may be used in any of the tensioner systems 4000B-F, I, M, and P-T described herein in place of the centralizing sleeve 4090.

With reference to FIGS. 4, 4D, and 4E, the tensioner system 4000B-F, I, M, and P-T further comprises a plurality of loading cups (also referred to as “upper impact buckets”) 4030A through 4030F connected to the tops of the respective cylinder rods 4015A-F. The loading cups 4030A-F are disposed within respective openings 4022A through 4022F of the load ring 4020. The loading cups 4030A-F transfer compression from the load ring 4020 to the cylinder rods 4015A-F. Thus, each tensioner system 4000B-F, I, M, and P-T transfers riser load from its respective riser 3010B-F, I, M, and P-T to its support frame 4040.

The tensioner systems 4000B-F, I, M, and P-T are designed to be capable of withstanding and operating through repeated cylinder bottom-out situations. To this end, each tensioner system 4000B-F, I, M, and P-T further comprises a plurality of stoppers (lower impact buckets) 4045A through 4045F secured to a top surface 4041 of the support frame 4040 so that respective cylinder rods 4015A-F pass there through. Each stopper 4045A-F comprises a respective shock absorbing element 4046A through 4046F. In an exemplary embodiment, the shock absorbing elements 4046A-F are formed from urethane (or other energy absorbing materials).

As each dry-tree unit 2010B-F, I, M, and P-T moves vertically in survival hurricane conditions, its respective tensioner system 4000B-F, I, M, and P-T, specifically the cylinders 4010A-F thereof, may be stroked downwardly to the down-stroke limit position, a “bottom-out” condition. This loading condition could result in a high energy impact load as the load ring 4020 comes in contact with the support frame 4040. The shock absorbing elements 4046A-F are included to absorb most of this impact load. Thus, the loading cups (upper impact buckets) 4030A-F are compressed against the stoppers (lower impact buckets) buckets 4045A-F during bottoming out. The shock absorbing elements 4046A-F absorb most of the impact load and transfer the respective riser 3010B-F, I, M, and P-T load from the cylinders 4010A-F to the upper cassette frame 4040 via the load ring 4020. The shock absorbing elements 4046A-F also allow for fabrication misalignment tolerances.

The tensioner systems 4000B-F, I, M, and P-T are also designed to be capable of withstanding and operating through repeated cylinder top-up situations. To this end, each cylinder 4010A-F comprises a respective shock absorber (or hydraulic cushion) 4016A through 4016F, as illustrated in FIG. 4F. The shock absorbers (or hydraulic cushion) 4016A-F are disposed within respective cylinders 4010A-F between an inner wall of the respective cylinders 4010A-F and an outer wall of respective cylinder rods 4015A-F. Each shock absorber (or hydraulic cushion) 4016A-F is located between a respective flange 4017A-F secured to a bottom end of the respective cylinder rod 4015A-F and a respective head 4014A-F secured to a head end of the respective cylinder rod 4015A-F. Each shock absorber 4016A-F may be a series of one or more energy absorbing springs, cushion material, or hydraulic cushion.

As a dry-tree unit 2010B-F, I, M, and P-T moves vertically in survival hurricane conditions, its respective tensioner system 4000B-F, I, M, and P-T, specifically the cylinders 4010A-F thereof, may be stroked upwardly to the up-stroke limit position, a “top-up” condition. This loading condition could result in a high energy impact load as the cylinder rods 4015A-F, specifically their respective flanges 4017A-F, come in contact with the cylinder head 4014 when the cylinder rod 4015 is extended to its maximum. The shock absorbers 4016A-F are included to absorb most of this impact load. Thus, the shock absorbers 4016A-F are compressed between the respective heads 4014A-F and flanges 4017A-F of the respective cylinders 4010A-F. The shock absorbers 4016A-F progressively decelerate the topping up motion in the last few inches of stroke and absorb/dissipate the impact energy as the tensioner systems 4000B-F, I, M, and P-T top up during survival conditions. The shock absorbers 4016A-F of each tensioner system 4000B-F, I, M, and P-T reduce the respective riser 3010B-F, I, M, and P-T impact load from the cylinders 4010A-F at the “top-up” condition. They protect the structural integrity of the cylinder heads 4014A-F from a sudden topping up impact load as a result of the cylinder pistons 4017A-F abruptly hitting their respective cylinder heads 4014A-F from inside during a top-up condition.

As illustrated, each tensioner system 4000B-F, I, M, and P-T comprises four upper centralizers 4060 mounted on a top surface of the upper cassette 4040 and four lower centralizers 4070 mounted on a top surface of the lower cassette 4050, although it is to be understood that the tensioner systems 4000B-F, I, M, and P-T are not so limited. Other numbers of upper centralizers 4060 and lower centralizers 4070 are contemplated. The centralizers 4060 and 4070 provide additional constraint on lateral force and bending moment on the tensioner systems 4000B-F, I, M, and P-T induced by the inertial force of the trees 2010B-F, I, M, and P-T or blow-out preventers placed at the top of the risers 3000B-F, I, M, and P-T. The centralizers 4060 transfer lateral forces and bending moments applied to the tensioner systems 4000B-F, I, M, and P-T to the wellbay 5000 via the upper cassette 4040 of each tensioner system 4000B-F, I, M, and P-T. The centralizers 4070 transfer lateral forces and bending moments applied to the tensioner systems 4000B-F, I, M, and P-T to a drop-down structure 5010 (described below) of the wellbay 5000. The centralizers 4060 and 4070 can be mounted directly to the top of the upper cassette 4040 and the lower cassette 4050, respectively, or mounted on a plate that is secured to the upper cassette 4040 and lower cassette 4050, respectively. In exemplary embodiments in which a centralizer mounting plate is used, the mounting plate is removable and installed with the tension joint 4090 of each tensioner system 4000B-F, I, M, and P-T at the middle of the upper cassette 4040 and lower cassette 4050 open to pass the subsea connector 3040B-F, I,M,P-T.

Each accumulator in the bank of high pressure accumulators 2020A and 2020B is connected to a high pressure side of the cylinders 4010A-F. The high pressure (HP) accumulators 2020A-B provide nitrogen gas pressure, for example, to each cylinder 4010A-F and are designed to provide the specified stiffness for the risers 3010B-F, I, M, and P-T. The HP accumulators 2020A-B are remotely mounted just outside the immediate wellbay 5000 area, and gas from them is piped to the cylinders 4010A-F of each of the tensioner systems 4000B-F, I, M, and P-T.

A small low pressure accumulator is connected to the head of each cylinder 4010A-F of each tensioner system 4000B-F, I, M, and P-T to accept the gas/fluid in the respective cylinders 4010A-F as it strokes. These low pressure accumulators are mounted local to each tensioner system 4000B-F, I, M, and P-T. With the passive nature of the tensioner systems 4000B-F, I, M, and P-T, simple local manual controls are provided for operation and monitoring. Additional monitoring to a remote control room is accomplished with pressure transmitters monitoring the operating pressure of each individual cylinder 4010A-F of each tensioner system 4000B-F, I, M, and P-T.

Each riser tensioner system 4000B-F, I, M, and P-T is operated through a control panel that manually monitors, controls and regulates the gas pressure in the high pressure accumulators 2020A-B and the low pressure accumulators individually for each of the cylinders 4010A-F of the tensioner systems 4000B-F, I, M, and P-T. Pressure transmitters connected to the high pressure accumulators 2020A-B allow for pressure monitoring at a remote location in a control room.

As discussed above, each riser tensioner system 4000B-F, I, M, and P-T is secured to the hull 1010 of the vessel 1000 at two interfaces: an upper interface and a lower interface. Illustrated in FIG. 2C is a cross-sectional view of a portion of the vessel 1000 and specifically the deck 2000 and wellbay 5000 thereof. The wellbay 5000 comprises a drop-down structure 5010 connected to the bottom surface 5003 of the wellbay 5000 structural support grid 5020. The drop-down structure 5010 provides the lower interface with the tensioner assemblies 4000B-F, I, M, and P-T in which the lower cassette 4050 of each tensioner assembly 4000B-F, I, M, and P-T is secured to the drop-down structure 5010. The drop-down structure 5010 absorbs the lateral forces and bending moments imparted on the tensioner assemblies 4000B-F, I, M, and P-T via the lower interfaces. Specifically, these forces and bending moments are passed to the drop-down structure 5010 via the centralizers 4070 and the lower cassette 4050 of the tensioner assemblies 4000B-F, I, M, and P-T. The upper interface is between the upper cassette 4040 of each tensioner system 4000B-F, I, M, and P-T and a wellbay structural beam slot 5050 (described below) in the wellbay structural support grid 5020.

The structural support grid 5020 of the wellbay 5000 supports the loads of the riser tensioner systems 4000B-F, I, M, and P-T. An important design consideration is accommodating loads when the total stroke of the risers 3010B-F, I, M, and P-T exceeds the down stroke limit of the riser tensioner systems 4000B-F, I, M, and P-T, thereby causing the riser tensioner systems 4000 to bottom out. In such a scenario, the loading cups (upper impact buckets) 4030A-F engage with the stoppers (lower impact buckets) 4045A-F of the tensioner systems 4000B-F, I, M, and P-T.

Complete retraction of the riser tensioner systems 4000B-F, I, M, and P-T may create an enormous load on the deck 2000 (specifically the deck structure 2005 and the structural support grid 5020 of the wellbay 5000) of the vessel 1000. One solution could be to increase the stroke of the riser tensioner systems 4000B-F, I, M, and P-T further. Increasing the stroke of the riser tensioner systems 4000B-F, I, M, and P-T, however, comes with extra cost, may pose challenges in designing the riser tensioner systems 4000B-F, I, M, and P-T to have sufficient strength, and increases the elevation of the wellbay 5000 and associated deck structure 2005 to provide for the necessary air gap between the bottom 2003 of the deck structure 2005 and the surface 351 of the water 350.

As illustrated in FIG. 2C, two sides of the wellbay 5000 are connected to overhead support structure beams for a drilling support structure 1110. Although structural bracing may be desirable, the other two sides of the wellbay 5000 are relatively open to the adjacent areas of the vessel 1000. This “open ends” design facilitates handling of the riser tensioner systems 4000B-F, I, M, and P-T and also natural ventilation across the production riser assemblies 3000B-F, I, M, and P-T within the wellbay 5000.

Another solution is to design the wellbay 5000 to accommodate the enormous load created by limited stroke length of the riser tensioner systems 4000B-F, I, M, and P-T when they bottom out. Illustrated in FIGS. 5A and 5B are perspective views of a portion of the deck structure 2005 and particularly of the wellbay 5000 structural support grid 5020, in accordance with an exemplary embodiment of the present invention. The wellbay 5000 comprises a grid of box girders 5030A through 5030H and box girders 5040A through 5040D. The box girders 5030A-H and 5040A-D form a plurality of slots 5050A through 5050U. In an exemplary embodiment, the box girders 5030A through 5030H and box girders 5040A through 5040D are 10 to 12 feet high to support heavy riser systems.

The wellbay 5000 structural support grid 5020 is connected to the deck structure 2005 so that a top 5001 of the structural support grid 5020 is recessed from the top 2001 of the deck structure 2005, and the bottom 5003 of the structural support grid 5020 is flush with the bottom 2003 of the deck structure 2005. This arrangement of the wellbay 5000 preserves the airspace between the deck structure 2005 and the surface 351 of the water 350 and provides sufficient space above the structural support grid 5020 for the cylinder rods 4015A-F to extend by their maximum lengths, e.g., 50 feet. Additionally, recessing the wellbay 5000 structural support grid 5020 reduces drilling floor elevation without interrupting motion of the tensioner systems 4000B-F, I, M, and P-T, thus minimizing the impact of drilling rig structure on construction, topside integration, global performance, stability and mooring line force. In an exemplary embodiment, the top 5001 of the structural support grid 5020 is recessed from the top 2001 of the deck structure 2005 by 23 to 25 feet.

The wellbay 5000 structural support grid 5020 comprises girder beams 5030A-H which are connected at one end to the deck beam 2030A and at their other end to the deck beam 2030B. The deck structure 2005 further comprises a plurality of transverse beams 2040A through 2040H and 2050A through 2050H. Disposed on the beams 2040A-G adjacent to the wellbay 5000 are tapered beams 2060A through 2060H, and disposed on the beams 2050A-G are tapered beams 2070A through 2070H. The tapered beams 2060A-H and 2070A-G taper from the top 5001 of the structural support grid 5020 to the top 2002 of the deck beams 2040A-G and 2050A-H.

The arrangement of the wellbay 5000 within the deck 2000 allows the bottom 2003 of the deck structure 2005 to be flush with the bottom 5003 of the wellbay 5000 (and its disposed structural support grid 5020). This arrangement simplifies construction of the deck structure 2005 of the semi-submersible vessel 1000. Furthermore, it enhances air gap for the semi-submersible vessel 1000 while reducing wave impact loading on the structure of the wellbay 5000 (via the structural support grid 5020). Additionally, it lowers the foundations of the riser tensioners 4000B-F, I, M, and P-T and overall height of the wellbay 5000, which reduces wind load on the drilling unit 1100 and improves stability and mooring performance of the vessel 1000.

The slots 5050A-U form the wellbay 5000 support structure grid 5020 (in a grid of 3×7 for this exemplary design). In the illustrated embodiment, twelve of the slots 5050B-F, I, M, and P-T are used for production risers and one of the slots 5050K is used for a drilling riser. The remaining grid openings 5050A, G, H, J, L, N, O and U are used for access and other functions. Other embodiments in which the support structural grid 5020 comprises more or fewer slots is contemplated.

Each production riser unit 3000B-F, I, M, and P-T and its respective tensioner system 4000B-F, I, M, and P-T is supported by the structural support grid 5020 in a respective riser slot 5050B-F, I, M, and P-T sized based on design requirements. In an exemplary embodiment, each slot 5050B-F, I-M and P-T is a 16-foot by 16-foot square (but the slot size can be adjusted larger or smaller as needed), measured on the center lines of the beams 5030B-G and 5040A-D. In such embodiment, the wellbay 5000 is created by repeating the 16-foot spacing between the wellbay structural support grid beam 5030B-G and 5040A-D centerlines.

In a further exemplary embodiment, the center slot 5050K is in many cases designed to be used for drilling purposes. Thus, the center slot 5050K and adjacent slots 5050D and 5050R are larger than the other slots 5050A-C, 5050E-J, 5050L-Q, and 5050S-U. In such embodiment, the center slot 5050K and the adjacent slots 5050D and 5050R may be between 20 feet by 20 feet to 25 feet by 25 feet, measured between the centerlines of adjacent beams 5030D-E and 5040A-D. The other slots 5050B-C, 5050E-F, 50501-J, 5050L-M, 5050P-Q and 5050S-T are sized to accommodate the upper cassettes 4040 of the tensioner systems 4000.

Referring now to FIGS. 5C and 5D, there is illustrated an exemplary embodiment of the slot 5050B and the cassette 4040 of the tensioner system 4000B, in accordance with an exemplary embodiment of the present invention. Although FIGS. 5C and 5D illustrate only the slot 5050B and the cassette 4040 of the tensioner system 4000B, it is to be understood that the other slots 5050C-F, I, M, and P-T and cassettes 4040 of the tensioner systems 4000C-F, I, M, and P-T may be constructed similarly. It is further to be understood that the slots 5050D, 5050K, and 5050R and their corresponding cassettes 4040 may be larger or smaller, although they may be constructed similarly.

The slot 5050B comprises a ledge 5051B disposed therein. In an exemplary embodiment, the ledge 5051B comprises horizontal members 5052B. Corner brackets 5053B may be used to support the ledge 5051B.

The ledge 5051B is recessed from the top surface 5001 of the wellbay 5000 structural support grid 5020 by a distance, d, that is equal to the height, h, of the upper cassette 4040. When disposed within the slot 5050B, the upper surface 4041 of the upper cassette 4040 is flush with the top surface 5001 of the wellbay 5000 structural support grid 5020. Thus, the recessed ledge 5051B provides for reduced overall tensioner assembly 4000B stack-up height, generates a flush arrangement on the wellbay 5000 structure foundation, provides a simple installation and retrieval of the tensioner assembly 4000B, evenly distributes riser 3010B load on the cylinders 4010A-F to the wellbay 5000 structure foundation, and eliminates fatigue issues at the tensioner assembly 4000B frame 4040 and wellbay 5000 structural interface (via the interface to wellbay slot 5050B).

The cassette 4040 is sized to interface with the slot 5050B. The cassette 4040B therefore transfers the load of the riser 3010B evenly. Additionally, because the cassette 4040B is seated on the ledge 5051B of the wellbay beams 5030B, 5030C, 5040C, and 5040D, but not welded thereto, installation is simplified and the fatigue in the connection interface between the cassette 4040 and the wellbay beams 5030B, 5030C, 5040C, and 5040D is reduced.

The tensioner systems 4000B-F, I, M, and P-T acts as a cantilevered structure when stroking out. As the cylinder rods 4015A-F of each tensioner system 4000B-F, I, M, and P-T move upward close to the up-stroke limit, they are potentially subject to lateral deflection due to inertia force induced on the tensioner systems 4000B-F, I, M, and P-T by motion of the vessel 1000. Specifically, the upper portion 4001 of each tensioner system 4000B-F, I, M, and P-T is potentially subject to lateral deflection. The lateral deflection, especially in severe storm events, could impair performance of the tensioner systems 4000B-F, I, M, and P-T. To provide lateral support of the upper portion 4001 of the tensioner systems 4000B-F, I, M, and P-T, a tensioner lateral stabilization system may be introduced, as discussed below with respect to FIGS. 6A through 6C.

Referring now to FIGS. 6A and 6B, there is illustrated an exemplary embodiment of a plurality of the tensioner lateral stabilization systems, generally designated as 6000B-F, I, M, and P-T, in accordance with an exemplary embodiment of the present invention. Each lateral stabilization system 6000B-F, I, M, and P-T is connected to the wellbay 5000 structure adjacent to its respective tensioner assembly 4000B-F, I, M, and P-T and is also connected to its respective tensioner assembly 4000B-F, I, M, and P-T.

Each tensioner lateral stabilization system (hereinafter also referred to as “stabilizer”) 6000B-F, I, M, and P-T comprises a pair of vertical supports 6050 and 6060. Each stabilizer 6000B-F, I, M, and P-T further comprises a pair of respective horizontal support members 6020 and 6030 connected together by one or more cross members 6025 and a stabilization ring 6040. A first end 6021 of the support member 6020 and a first end 6031 of the support member 6030 is connected to the stabilization ring 6040. A second end 6022 of the support member 6020 is connected to the vertical support 6050, and a second end 6032 of the support member 6030 is connected to the vertical support 6060.

The stabilization ring 6040 is disposed around its respective riser 3010B-F, I, M, and P-T. The pair of vertical supports 6050 and 6060 are secured to the wellbay 5000 structural support grid 5020 adjacent to the respective tensioner assembly 4000B-F, I, M, and P-T. The upper end of the vertical supports 6050 and 6060 are fixed to the drilling support structure 1110 overhead or a structural member of the wellbay 5000. At the end 6022 of the support member 6020 is a trolley 6024 that engages with the vertical support 6050, and at the end 6032 of the support member 6030 is a trolley 6034 that engages with the vertical support 6060. In an exemplary embodiment, the vertical supports 6050 and 6060 are rails on which the trolleys 6024 and 6034 ride.

The attachment of the trolleys 6024 and 6034 to the vertical supports 6050 and 6060 in each stabilizer 6000B-F, I, M, and P-T prevents rotation of the stabilizers 6000B-F, I, M, and P-T 6000B-F, I, M, and P-T, while allowing the trolleys 6024 and 6034 to move up and down as the tensioner assemblies 4000B-F, I, M, and P-T move up and down. Thus, the stabilizers 6000B-F, I, M, and P-T provide lateral support to the risers 3010B-F, I, M, and P-T throughout the full range of motion of the cylinders 4015A-F. In an exemplary embodiment, the trolleys 6024 and 6034 employ roller bearings or sliding bearings for movement along the respective vertical supports 6050 and 6060.

The vertical supports 6050 and 6060 are attached to the wellbay 5000 structure directly or through stays, depending on application requirements. The vertical supports 6050 and 6060 and the horizontal support members 6020 and 6030 may be formed from aluminum or steel, depending on application requirements.

Referring now to FIG. 6C, there is illustrated an exemplary alternative embodiment of the stabilizers 6000B-F, I, M, and P-T, which exemplary alternative embodiment is generally designated as 6000B′-F′, I′, M′, and P′-T′ (hereinafter designated as 6000′ for brevity), in accordance with an exemplary embodiment of the present invention. Each stabilizer 6000′ comprises, respectively, a pair of respective horizontal support members 6020′ and 6030′. The support members 6020′ and 6030′ are connected together by one or more respective cross members 6025′ and 6035′.

A first end 6021′ of each support member 6020′ and a first end 6031′ of each support member 6030′ are secured to a respective platform 2090B-F, I, M, and P-T. Such attachment prevents rotation of the stabilizers 6000′. A second end 6022′ of the support member 6020′ is connected to the vertical support 6050, and a second end 6032′ of the support member 6030′ is connected to the vertical support 6060. At the end 6022′ is a pair of trolleys 6024′ and 6026′ that engage with the vertical support 6050, and at the end 6032′ is a pair of trolleys 6034′ and 6036′ that engage with the vertical support 6060.

The trolleys 6024′, 6026′, 6034′, and 6036′ move up and down as the respective tensioner assembly 4000B-F, I, M, and P-T moves up and down. Thus, they provide lateral support to the riser 3010B-F, I, M, and P-T throughout the full range of motion of the cylinders 4015A-F. In an exemplary embodiment, the trolleys 6024′, 6026′, 6034′, and 6036′ employ roller bearings or sliding bearings for movement along the respective vertical supports 6050 and 6060.

In another exemplary embodiment, the trolleys of either of the stabilizers 6000 or 6000′ are designed to have a gap between them and their respective vertical supports 6050 and 6060. In such embodiment, the stabilizers 6000 or 6000′ arrest horizontal bending of the tensioner assemblies 4000B-F, I, M, and P-T only when large unwanted deflections occur.

The tensioner assemblies 4000B-F, I, M, and P-T and the wellbay 5000 provide a feasible and cost-effective solution for deep and ultra-deep water field development using top-tensioned risers 3010B-F, I, M, and P-T on a floating system, such as the semi-submersible vessel 1000, a spar platform, a TLP, and other floaters. This design provides not only a more efficient concept for the production units 3000B-F, I, M, and P-T and the drilling unit 1100, but also especially enable a floater with larger motion characteristics applicable to deep/ultra-deep water field development. For example, the vessel 1000, a dry tree semi-submersible, comprising the wellbay 5000 and long stroke riser tensioner systems 3000B-F, I, M, and P-T is able to overcome the challenges due to large platform motion at severe field conditions, thus enable the dry tree semi-submersible for deep/ultra-deep water field development. In addition, the tensioner assemblies 4000B-F, I, M, and P-T are able to compensate long stroke of risers due to significant thermal expansion during deep water and ultra-deep water extraction, which is extremely valuable for high temperature field development.

These and other advantages of the present invention will be apparent to those skilled in the art from the foregoing specification. Accordingly, it is to be recognized by those skilled in the art that changes or modifications may be made to the above-described embodiments without departing from the broad inventive concepts of the invention. It is to be understood that this invention is not limited to the particular embodiments described herein, but is intended to include all changes and modifications that are within the scope and spirit of the invention. 

What is claimed is:
 1. A riser tensioner system comprising: a first cassette; a second cassette; a tension joint disposed through the first cassette and the second cassette; one or more centralizers providing lateral support to the tension joint; and a plurality of cylinders, each comprising a first end, a second end, and an intermediate portion, wherein the first end of each of the plurality of cylinders is secured to the first cassette and the intermediate portion of each of the plurality of cylinders is secured to the second cassette.
 2. The riser tensioner system of claim 1, wherein the one or more centralizers comprise one or more first centralizers and one or more second centralizers, the one or more first centralizers secured to the first cassette, the one or more second centralizers secured to the second cassette, wherein the one or more first centralizers and the one or more second centralizers provide lateral support to the tension joint.
 3. The riser tensioner system of claim 1, further comprising a load ring attached to a top of the tension joint, wherein the plurality of cylinders are coupled to the load ring to apply tension thereto to apply tension to the tension joint.
 4. The riser tensioner system of claim 3, wherein each of the plurality of cylinders comprises a cylinder rod, a top of which is coupled to the load ring to apply the tension thereto to apply the tension to the tension joint.
 5. The riser tensioner system of claim 4, further comprising a first plurality of impact buckets or loading cups secured to the load ring and a second plurality of impact buckets or stoppers secured to the first cassette.
 6. The riser tensioner system of claim 5, wherein the top of the cylinder rod of each of the plurality of cylinders is secured to a respective one of the first plurality of impact buckets.
 7. The riser tensioner system of claim 5, wherein the cylinder rod of each of the plurality of cylinders is slidably disposed along a cylinder axis through to a respective one of the second plurality of impact buckets, and slidably slipped perpendicular to the cylinder axis within a respective one of the first plurality of loading cups
 8. The riser tensioner system of claim 5, wherein the first plurality of impact buckets and the second plurality of impact buckets are positioned to impact one another during a bottom-out condition of the riser tensioner system.
 9. The riser tensioner system of claim 1, wherein each of the plurality of cylinders comprises an interior shock absorber for absorbing an impact load resulting from a top-up condition of the riser tensioner system.
 10. The riser tensioner system of claim 1, wherein the first cassette comprise a top surface and a bottom surface, and wherein the first cassette is sized to be disposed within an opening in a wellbay so that the top surface of the first cassette is flush with a top surface of a structural support foundation of the wellbay.
 11. The riser tensioner system of claim 10, wherein the second cassette is securable to a lower support structure of the wellbay.
 12. The riser tensioner system of claim 1, further comprising a stabilization system comprising: at least one vertical support; and at least one horizontal support, a first portion of which is disposed around the riser and a second portion of which is slidably connected to the at least one vertical support.
 13. The riser tensioner system of claim 12, wherein the at least one horizontal support comprises at least one respective tensioner ring that is disposed around the riser.
 14. The riser tensioner system of claim 1, further comprising a stabilization system comprising: at least one vertical support; a platform disposed around the riser; and at least one horizontal support, a first portion of which is connected to the platform and a second portion of which is slidably connected to the at least one vertical support.
 15. A wellbay structure for use in an offshore platform, the wellbay structure comprising: a plurality of transverse girders; and a plurality of longitudinal girders, wherein the plurality of transverse girders and the plurality of longitudinal girders form a grid comprising a plurality of slots, and wherein each of the plurality of slots is configured for receiving and supporting a first cassette of each of a respective one of a plurality of riser tensioner assemblies.
 16. The wellbay structure of claim 15, wherein each of the plurality of transverse girders and each of the plurality of longitudinal girders are box girders.
 17. The wellbay structure of claim 15, wherein the grid comprises a top and a bottom, and wherein the bottom of the grid is flush with a bottom of a lower level deck of the offshore platform.
 18. The wellbay structure of claim 15, wherein the grid comprises a top and a bottom, and wherein the wellbay structure further comprises a drop-down structure attached to the bottom of the grid.
 19. The wellbay structure of claim 18, wherein the drop-down structure is configured for supporting a second cassette of each of the plurality of riser tensioner assemblies.
 20. The wellbay structure of claim 15, wherein each of plurality of slots comprises a ledge for supporting the first cassette of a respective one of the plurality of riser tensioner assemblies.
 21. The wellbay structure of claim 20, wherein the ledge in each of the plurality of slots is recessed from a top surface of the grid.
 22. The wellbay structure of claim 20, wherein the ledge in each of the plurality of slots comprises at least two or more horizontal members disposed on adjacent ones of the plurality of the plurality of transverse girders or adjacent ones of the plurality of longitudinal girders.
 23. An offshore floating platform comprising: an upper deck; a lower deck; and a wellbay structure comprising: a plurality of transverse girders; and a plurality of longitudinal girders, a plurality of riser tensioner assemblies, each comprising: a first cassette; a second cassette; a tension joint slidably disposed through the first cassette and the second cassette; one or more centralizers providing lateral support to the tension joint; and a plurality of cylinders, each comprising a first end, a second end, and an intermediate portion, wherein the first end of each of the plurality of cylinders is secured to the first cassette and the intermediate portion of each of the plurality of cylinders is secured to the second cassette, wherein the plurality of transverse girders and the plurality of longitudinal girders of the wellbay structure form a grid comprising a plurality of slots, and wherein each of the plurality of slots is configured for receiving and supporting the first cassette of a respective one of the plurality of riser tensioner assemblies.
 24. The offshore floating platform of claim 23, wherein each of the plurality of transverse girders and each of the plurality of longitudinal girders of the wellbay structure are box girders.
 25. The offshore floating platform of claim 23, wherein the grid of the wellbay structure comprises a top and a bottom, and wherein the bottom of the grid is flush with a bottom of the lower deck.
 26. The offshore floating platform of claim 23, wherein the grid of the wellbay structure comprises a top and a bottom, and wherein the wellbay structure further comprises a drop-down structure attached to the bottom of the grid.
 27. The offshore floating platform of claim 26, wherein the drop-down structure is configured with limited deformation for supporting a second cassette of each of the plurality of riser tensioner assemblies.
 28. The offshore floating platform of claim 23, wherein each of plurality of slots comprises a ledge for supporting the first cassette of a respective one of the plurality of riser tensioner assemblies.
 29. The offshore floating platform of claim 28, wherein the ledge in each of the plurality of slots is recessed from a top surface of the grid.
 30. The offshore floating platform of claim 28, wherein the ledge in each of the plurality of slots comprises at least two or more horizontal members disposed on adjacent ones of the plurality of the plurality of transverse girders or adjacent ones of the plurality of longitudinal girders.
 31. The offshore floating platform of claim 23, wherein the platform is capable to apply to dry tree production and/or drilling operations in deep or ultra-deep water field development. 